Fossiliferous basalt

from the north coast of Iceland

Figure 1, above: a small sample with distinctive heft, remarkable for containing white
mollusc (bivalve) shells in a granular matrix.
The fossil debris, virtually all bivalve shells, occurs in a very fine-grained,
chilled, granular dark brown basaltic host.
One 3-mm coiled fragment may be part of a gastropod shell.
So, who am I kidding? Well:

The term "fossil": pretty youthful by fossil standards -
the whole of Iceland and adjacent ocean floor is
geologically recent, a part of the Tertiary Volcanic Province
formed as the north Atlantic ocean opened.
The shells are of Pliocene (5.3-2.6 million years) or more likely
Quaternary (Pleistocene-Holocene, <2.6 million years) age.
Nevertheless, a most impressive mode of preservation!
A relatively "old" rock from Iceland is the late Miocene, silicic
Slaufrudalur stock, age dated at 6.6 Ma (Carmody et
al., 1992).
Even the oldest rocks found on Iceland, generally
on the west and east coasts furthest from the main rift zone,
may be only 16 million years or so in age,
erupted / deposited in early to middle Miocene time.

The term "basalt":
the host rock to the shells is very granular, and possibly an extremely
immature, poorly sorted sediment of grains derived from
basalt volcanism, composed of dense, dark
minerals such as pyroxenes, magnetite and ilmenite.
The sample is not glassy in appearance, and does not seem to be
a quenched basaltic melt formed from submarine eruption
into sea water. It may represent a
lithified basaltic tuff, an ash fall that engulfed
the shells.

The sample, "PB14", was collected from the Tjornes area by Doug Astill in August 1969.
7.0x4.5x2.0 cm in size, it is less magnetic than one might expect of fresh basalt,
possibly by alteration of magnetite during cooling (?). Bulk magnetic susceptibility
is about 1.9x10-3 SI units.
When first received it was weighed at 52.22 grams. However, tipped off that it seems to absorb water
readily, it was heated for 30 minutes at about 115°C (240°F) and reweighed at
51.67 g, a loss of 1 percent. Within 24 hours it had regained
almost all the loss, to 52.15 g.

"Rock of the Month #141, posted for March 2013" ---

"Fossiliferous basalt" sounds like an impossibility:
one can reasonably expect a hot silicate lava to melt
and absorb carbonate-dominated shells the same way
butter melts on hot peas!
Can we hedge our bets, and
avoid undertaking destructive
sampling of the specimen? The extreme case would be
the shells becoming entrained in rapidly-cooling lava,
cooling before the shells could be destroyed.
Maybe! Perhaps a basalt flow was eroded on the Icelandic coast, and the
disintegration of the lava produced a coarse sand that
incorporated some shells and was then preserved, perhaps as an
interflow sediment armoured by the arrival of a younger flow, which
preserved the rock.
Another alternative is that the sample represents not lava but a
volcanic ash deposit, a layer of basic tuff interbedded with the lavas, and which
engulfed some shells along the shoreline.
A thin section view would provide additional textural evidence.
Now I want to visit Tjornes, too!

The Tjornes peninsula displays as many as 12 lithological cycles
along the coastal sections, each beginning with a diamictite
(a poorly-sorted terrestrial sediment, here
interpreted as a lithified glacial deposit)
overlain by further terrestrial sediments and lava flows.
Interbedded fossiliferous marine mudrocks and sandstones
indicate repeated marine incursions, followed by further
diamictites. Tuff layers are present, as well as volcaniclastic
sedimentary layers and lava flows (Eiriksson, 1981).

The whole of Iceland is displayed in series of nine maps
at 1:250,000 scale, in both topographic and geological
series. The map areas of interest here are sheets 4 and 7, mid-north and
northeast Iceland, respectively.
The maps viewed for this note were published by the Geodetisk Institute
of Copenhagen in 1966.
The exact provenance of the specimen is lost to us.
It is most probably from the
north-facing coast of Sjavarsandur, south
of the bay of Skjalfandi, on the west side of the
Tjornes peninsula,
south of Husavik and northwest of Myvatn (a town, located inland
to the south of the peninsula).

The sediments of the Tjornes peninsula
have been assigned an upper Pliocene to Pleistocene age
(Einarsson et al., 1967).
Marine and non-marine sediments, intercalated with basalt flows
and, higher up, with tillite layers representing at least 10 glacial
episodes, offer a tantalising record of the Ice Age history of the
northern hemisphere, more complete than previously noted in
North America and Europe.
The evidence of as many as 10 glaciations suggests that the
Bering Land Bridge, thought critical
in the peopling of the Americas,
may have been renewed as many as ten times by glacial-eustatic
sea level lowerings (Einarsson et al., 1967).
Detailed studies of bivalves, echinoids and brachiopods
suggest periodic Arctic linkages between the Pacific and Atlantic
oceans (Durham and MacNeil, 1967). The molluscs in the Tjornes
Beds include bivalve genera such as Glycimeris, found also in the
sandy Red Crag deposits of Essex,
northwards into east Anglia (the counties
of Suffolk and Norfolk), in southeast England.
The late Tertiary and Quaternary paleoecology of that region
of Britain has
been documented in great detail since at least the 19th century, and
some of the more-recent work can be found in: Greensmith
et al. (1973); Hunt (1989; Head (1998); as well as
a non-technical synthesis of Essex geology by Lucy
(1999).

The mid-ocean-ridge volcanism, as in Hawaii,
has been linked to the upwelling of a deep mantle plume
(Schilling et al., 1999).
The Tjornes district is seen as a fracture system, part of the
larger-scale rifting of Iceland, and the wider Atlantic, bisecting the
island in a broadly southwest to northeast direction (Saemundsson,
1974).

The local geology has been well-researched, being
such an ever-present, active thread in the
fabric of Icelandic life.
The wild landscapes of Iceland and Scandinavia influenced
Old Norse mythology (Bergstrom, 1989).
To this day the scenery captivates and draws tourists from
around the world (e.g., Winter, 1997; Hill, 2011; Kunzig et
al., 2012).

An Icelandic contribution to geology,
beyond the ubiquitous volcanic features and hot springs,
is the sandur (plural sandar), alluvial outwash plains
deposited by streams of meltwater flowing away from the front of
a glacier
(Bluck, 1974; Maizels, 1993; Zielinski and Van Loon, 2003).
They are known elsewhere in the world, including Poland
and (Church, 1972) Baffin Island in the Canadian Arctic.

Postscript:
two polished thin sections were prepared from an
offcut sliver of the sample, as shown in the
following images.

The two photomicrographs display the shell fragments in a fine-grained groundmass
of igneous crystals, mostly 0.1 mm or less in diameter.
These include olivine, clinopyroxene and plagioclase
feldspar in an oxidized matrix. Some of the olivine is
altered to dark brown "iddingsite". Most
notably, the shells are not the only foreign ingredients:
the rock also contains rounded carbonate grains and
scattered grains of quartz,
which is out of chemical equilibrium with the olivine of the host rock.
The nature of the groundmass which cements the crystals and shells is
critical to the interpretation. If it is a devitrified glass
then the rock is properly a lava.
However, if the scattered voids are vesicles (gas bubbles)
and the matrix a very fine-grained dust or silt,
then the rock may be a tuffaceous deposit.
Upon further examination, the sample contains about 80%
magmatic components, including what appears to be a partially
devitrified basaltic glass (palagonite) and abundant
sheet silicate minerals, including serpentine replacing olivine,
as well as various magmatic minerals found in basalt, such as
pyroxenes, olivine, plagioclase feldspar and Fe-Ti oxides.
The remainder of the rock seems to have been scoured
from a beach or older beach deposit: quartz (sand grains),
shells, and rounded carbonate blobs, perhaps derived from
broken shells. The quartz remains more angular than the carbonate,
perhaps because it is more refractory, and slower to alter
in proximity to lava.
So, if the identification of a quenched glass is sound, this
is
indeed a "fossiliferous basalt", and
at the least, a
"fossiliferous basaltic tuff".

Acknowledgements: The sample, maps and memories were all
proffered by Doug Astill, who explored this fascinating
land in the summer of '69! The thin section, of a tricky material,
was rendered in expert fashion by Anne Hammond.

Einarsson,T, Hopkins,DM and Doell,RR (1967) The stratigraphy
of Tjornes, northern Iceland, and the history of the Bering land
bridge. In `The Bering Land Bridge' (Hopkins,DM editor),
Stanford University Press, 495pp., 312-325.